1. Field of the Invention
The present invention generally relates to metrology of linewidths, trenches and overlays on a submicron range using strain sensors and, more particularly, to probe assembly used in combination with an Atomic Force Microscope (AFM) for making measurements in three dimensions. The AFM is used to make measurements in the z direction and then the probe assembly is used in the x-y plane perpendicular to the z direction to profile a sample.
2. Description of the Prior Art
Quality control in the manufacture of integrated circuits (ICs) requires a technology for measuring surface geometries of the IC wafer. One such technology employs a small thin piezoelectric member affixed to a larger mounting member to which there is attached a probe tip. The probe tip is made to contact the surface being mapped thereby causing a flexing of the mounting member and a corresponding flexing of the piezoelectric member, producing an electrical signal. An example of this technology is shown in U.S. Pat. No. 4,888,550 to Reid which discloses a multiprobe tip assembly used for quality control testing of an integrated circuit wafer. The Reid device uses a strain sensing piezoelectric element at the base supporting the entire tip structure and, because of this construction, is limited to sensing only in the vertical direction.
In practice, the contact tip technology as known in the prior art has been useful in measuring micron scale or larger structures. Furthermore, the sensitivities of these structures has been relatively low, making them unsuitable for measurements of submicron scale structures such as those of the newer gigabit chips.
Scanning Tunneling Microscopy (STM) technology introduced a new concept in measurements using a probe type approach to map the surfaces of integrated circuit structures. The basic STM is disclosed in U.S. Pat. No. 4,343,993. Briefly described, a sharply pointed, electrically conductive tip is placed at a distance on the order of one nanometer from the conductive surface of a sample to be investigated, with an appropriate electrical potential applied across the gap between the tip and surface. As the electron clouds of the atoms at the apex of the tip and the surface touch, a flow of electrons will result giving rise to a tunneling current which happens to be extremely sensitive to changes in gap width. To render these changes as close as possible to zero, a feedback control system controls the distance of the tip from the surface, using deviations of the tunneling current from an initial value as a control signal. This control signal is also employed to generate a plot of the topography of the surface being investigated.
Atomic Force Microscopy (AFM) is a variation of the STM technology. In one design, a sensor consisting of a spring-like cantilever which is rigidly mounted at one end and carries at its free end a dielectric tip, profiles the surface of an object. The force between the object's surface and the tip deflects the cantilever, and this deflection can be accurately measured. A spatial resolution of 3 nm has been achieved.
Another version of the AFM includes optical detection instead of STM detection. In this version, a tungsten tip at the end of a wire is mounted on a piezoelectric transducer. The transducer vibrates the tip at the resonance frequency of the wire which acts as a cantilever, and a laser heterodyne interferometer accurately measures the amplitude of the a.c. vibration. The gradient of the force between the tip and the sample modifies the compliance of the lever, hence inducing a change in vibration amplitude due to the shift of the lever resonance. Knowing the lever characteristics, one can measure the vibration amplitude as a function of the tip-to-sample spacing in order to deduce the gradient of the force and, thus, the force itself.
A most critical component of the AFM is the spring-like cantilever. As a maximum deflection for a given force is needed, a cantilever is required which is as soft as possible. At the same time, a stiff cantilever with a high natural frequency is necessary in order to minimize the sensitivity to vibrational noise from the building. For meeting both requirements, dimensions of the cantilever beam are necessary that can only be obtained by microfabrication techniques. Another most critical component of the AFM is the tip itself. Tip "crashes" are the most likely failure cause of AFM tips, and such failures are catastrophic, requiring the replacement of the cantilever beam and tip subassembly. This is perhaps the greatest limitation to the use of AFM.
While STM and AFM both have the potential capability for profiling on an atomic diameter scale, the difficulty with both is the measurement of trenches in two dimensions. Both require sophisticated multi-connected interferometers to detect the two-dimensional tip motion and, for AFM to be useful as a reliable and accurate metrology tool, the sensor tip should be highly reproducible in the fabrication process and the tip shape should be known with high accuracy. Moreover, for profiling and critical dimension (CD) measurement of silicon (Si) memory trenches with submicron trenches with depths of 1 .mu.m or more, nanometer-scale tip shapes with high aspect ratio will be required.